This poem expresses Yeats's relentlessly fatalistic view of warfare in general and particularly WWI.
The poem is framed as the internal monologue of an Irish airman, and we hear a man who not only knows he is going to die "among the clouds above," but who doesn't seem to care. He is under no illusions, for example, of fighting and dying for duty, honor and country when he acknowledges that he neither hates his enemy nor loves his countrymen ("those that I guard"). Nor is fighting for Ireland or England--his country is limited to "Kiltartan Cross" and his countrymen are "Kiltartan's poor," a very localized conception of one's country.
The airman's own fatalism is embodied in his statement that his death will not make his countrymen feel a loss, and it certainly won't make them happier. And, just in case the reader is under any illusions about the airman's reasons for going to war, the airman notes that no law or duty "bade me fight." He simply wanted to fly.
In the end, he made his decision based on a balanced assessment of what kind of life the future held for him, which "seemed a waste of breath," and the life he left behind, which he also characterizes as a "waste." One might argue with his assessment of the past and future, but one cannot argue that, from his perspective, he has weighed his life and decided the "impulse of delight" that drives him into the clouds hat tipped the balance.
What makes him an interesting speaker is his very logical, but truly fatalistic, view of his circumstances, and we have to acknowledge that, based on his view of life, his decision appears to be a rational one. The immediate joy of flying simply outweighs the grim past and future he sees before him.